Soap chemical stops fish sticking together

Fish form shoals to defend themselves against predators and to find food faster

(Image: A. Ward)

A contaminant found in rivers and estuaries the world over can “rob” fish of their ability to sense each other and stay in a tight, cohesive shoal, say researchers.

The chemical, 4-nonylphenol, does this by overpowering the fish’s natural smell-signatures, say researchers. And because these signatures are critical to helping the fish form in groups, the chemical effectively weakens their “strength in numbers” defence against predators.

“The loss of the ability to shoal cohesively is serious business for fish. It’s a defensive strategy. If fish can’t shoal properly, they are extremely vulnerable to predation,” says Ashley Ward at the University of Sydney, Australia, who led the study.

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Nonylphenol or 4-NP is widely used in soaps, sewage treatment, and in some pesticides. They are known to affect human and animal hormonal systems, and can “feminise” fish, causing males to produce typically female proteins.

In developed nations, the maximum concentration deemed “permissible” is between 0.5 and 1 microgram per litre of water, because fish do not show signs of stress at this level. In European rivers, typical concentrations range from 0.1 to 340 micrograms per litre.

Keeping their distance

Ward and colleagues decided to test whether “permissible” levels of 4-NP could disrupt social organisation in banded killifish, a shoaling fish commonly found in North American lakes.

Ward – then a researcher at Mount Alison University in New Brunswick province, Canada –gathered killifish from the nearby Morice Lake. He found that the groups of fish that were placed in aquariums with 1 microgramme per litre of 4-NP tended to stay at least twice as far from each other as those in uncontaminated aquariums.

The chemical did not appear to affect the fish’s ability to smell other substances, as they were just as able to find food hidden away in their aquarium as the fish in the uncontaminated tanks.

Other experiments suggested that the reason the fish shoals were not as tightly grouped in the presence of 4-NP was that the chemical was masking the fish’s own smell. “Shoaling fish develop a chemical profile based on their recent habitat and diet – they smell of what they eat and where they have been, just like us,” explains Ward. “They prefer to shoal with fish that smell similar to themselves.”

Fish are also thought to produce chemical signals relating to their social dominance, reproductive state, and genetic make-up. The last type of signals help them avoid breeding with relatives.

‘Sub-lethal’ effects

But 4-NP is a lipophillic compound, meaning it tends to stick to oily surfaces – a fish, for example. “It seems that it might ‘coat’ the fish,” says Ward. This changes their individual chemical signature and breaks down recognition among the fish.

Ward and his colleagues placed single killifish in a corridor of water that had two separate currents running down it. One current was clean. The other came from a tank in which the researchers had placed killifish that had previously bathed for an hour in 4-NP contaminated water.

The team found that the lone killifish moved away from the contaminated current when the upstream fish had bathed in 4-NP at concentrations of 1 microgram per litre or higher.

“I think we need to reappraise our comfortable position that if a certain concentration doesn’t actually kill an animal, it’s OK,” Ward told New Scientist. “There are subtle ‘sub-lethal’ effects that can be devastating in the medium and long term.”

Ward and his team point out that other chemicals, heavy metals for example, damage the olfactory organs of fish. They say that in polluted waters, chemicals like 4-NP and heavy metals could both be present, one affecting the way that fish smell, the other their ability to smell.